A number of processes have been proposed for the reduction of NO.sub.x and SO.sub.x generated by the combustion of coal in a boiler. Essentially, those processes that have been devised in the past are satisfactory for reducing either of the NO.sub.x and SO.sub.x compounds but not for the reduction of both.
It has been proposed to condition the combustion process so as to reduce the amount of NO.sub.x formed. New burners are commercially available that modify the method of mixing in the fuel to achieve 20% to 50% NO.sub.x reduction. In another method, selective catalytic reduction (SCR) reduces NO.sub.x after it has formed by reacting with ammonia (NH.sub.3) in the presence of a catalyst to reduce NO to N.sub.2 and water. The SCR method is capable of achieving up to 90% NO.sub.x removal. In selective noncatalytic reduction (SNR), either NH.sub.3 or urea is injected into the boiler at high temperatures to reduce NO.sub.x to N.sub.2 and water without a catalyst. Representative patents which disclose the SNR method discussed above is U.S. Pat. No. 4,208,386 to Arand et al and U.S. Pat. No. 4,636,370 to Dean et al. Again, while the processes referred to above have been employed for many years to reduce NO.sub.x certain problems have been encountered. For example, combustion modification is relatively inexpensive but is limited in the amount of reduction that can be realized. SCR also has been successful, to some degree, but the catalyst employed often becomes contaminated with SO.sub.x and requires replacement after as little as two years of use. Certain catalysts are classed as hazardous waste materials and thus the process is expensive both to install and operate. Furthermore, SNR has been more successful when used for the removal of NO.sub.x in gas turbines and other low NO.sub.x units but has definite limitations when used in the reduction of NO.sub.x in coal-fired boilers and especially those employed at electrical power stations.
Both the SCR and SNR processes have demonstrated a tendency to generate new pollutants during the NO.sub.x removal reaction. On account of the inefficiency of the reaction, some ammonia can be generated and is emitted with the other combustion effluent. In U.S. Pat. No. 4,780,289 to Epperly et al, a method is disclosed for the reduction of ammonia (NH.sub.3) emissions by controlling the reaction temperature very carefully and asserts that it is possible to reduce NH.sub.3 from 50 ppm to below 10 ppm; however, close constant control is required to regulate the process parameters in maintaining NH.sub.3 emissions. In U.S. Pat. No. 4,777,024 to Epperly et al, it is proposed to inject a second treatment agent to limit ammonia (NH.sub.3) emissions. Various chemicals, such as, oxygenated hydrocarbons, amino acids, proteins and methyl compounds are proposed for use as the second treatment agent. It is asserted that it is possible to realize up to 80% NO.sub.x reduction with ammonia slips of 21 ppm but again requires extremely close control over the urea injection process and amount of treatment agents to limit the amount of ammonia emissions.
Other systems have been proposed for the removal of SO.sub.2 pollutants. Wet limestone scrubbers have been employed on high sulfur coal with reductions of SO.sub.2 up to 90% being claimed. It has also been proposed to inject limestone into the boiler to reduce the amount of SO.sub.2 pollutants. To the present time, these technologies largely have been proposed for use independently of the reduction of NO.sub.x. When sodium injection has been employed in the past, it has offered the potential for high SO.sub.2 removal in relation to installed costs; however, in the process of converting the SO.sub.2 gas to sodium sulfate which is a solid collected with the fly ash, a portion of the NO is converted to NO.sub.2 and in sufficient quantities can result in the emission of a visible plume from the stack of the boiler.
U.S. Pat. No. 4,844,915 to Hooper discloses a process wherein urea and a sodium reagent are intermixed, injected simultaneously and maintained in contact with the flue gas for a time period sufficient to react the sodium reagent with at least some of the SO.sub.x and NO.sub.x to increase both the percentage SO.sub.x and NO.sub.x removal while suppressing the conversion of NO to NO.sub.2. Lacking in Hooper is a recognition of the amount of ammonia slip produced as a result of the urea injection and the fact that there is no change in NO removal as a result of the addition of urea. Broadly, from the technologies evaluated in this field, there continues to be a significant problem in achieving high NO.sub.x removal without high ammonia (NH.sub.3) emissions or in obtaining high SO.sub.2 removal without creating a visible NO.sub.2 plume. It is therefore proposed in accordance with the present invention to
achieve substantial reductions of NO.sub.x and SO.sub.x and specifically to substantially reduce the formation of NO.sub.2 to a level which will avoid formation of a visible brown plume.